This practice has some interesting historical antecedents. The main species of nitrogen in mammal urine is urea (H2N-C-NH2), but the ideal form of nitrogen for fertilizer is probably nitrate (NO3-) in most cases (and it's easier to store and transport as a solid). Soil bacteria in healthy soils around roots will also transform urea to ammonia and nitrate in fairly complicated processes, and there are pH effects. For an overview:
However, nitrate has many other practical uses, and in industrial Europe before the invention of Haber-Bosch, a major use was gunpowder manufacture (the ingredient saltpeter, potassium nitrate KNO3). An entire industry existed, based on taking animal and human urine (and other materials, like rotting animal carcasses) and processing it in a certain manner to encourage its conversion to nitrate crystals, typically involving beds of wood chips which would become covered with a frost of potassium nitrate crystals. The British government passed laws about the storage of urine for the king's 'petermen' to come and collect for this purpose. Here's a technical manual from 1862 on the process, from start to pure potassium nitrate (which in turn, could be converted to the important industrial chemical, nitric acid):
https://www.cannagardening.com/how_ammonium_nitrate_ratio_af...
However, nitrate has many other practical uses, and in industrial Europe before the invention of Haber-Bosch, a major use was gunpowder manufacture (the ingredient saltpeter, potassium nitrate KNO3). An entire industry existed, based on taking animal and human urine (and other materials, like rotting animal carcasses) and processing it in a certain manner to encourage its conversion to nitrate crystals, typically involving beds of wood chips which would become covered with a frost of potassium nitrate crystals. The British government passed laws about the storage of urine for the king's 'petermen' to come and collect for this purpose. Here's a technical manual from 1862 on the process, from start to pure potassium nitrate (which in turn, could be converted to the important industrial chemical, nitric acid):
https://docsouth.unc.edu/imls/lecontesalt/leconte.html
Whether or not this widespread practice had anything to do with the origin of the British phrase 'taking the piss' is a mystery to me.